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United States Patent |
5,045,278
|
Das
,   et al.
|
September 3, 1991
|
Dual processing of aluminum base metal matrix composites
Abstract
An aluminum based metal matrix composite is produced from a charge
containing a rapidly solidified aluminum alloy, a carbidiferous agent and
particles of a reinforcing material present in an amount ranging from
about 0.1 to 50% by volume of the charge. The charge is ball milled
energetically to uniformly mix the carbidiferous agent within the aluminum
matrix, and to enfold metal matrix material around each of the particles
while maintaining the charge in a pulverulent state. Upon completion of
the ball milling step, the charge is hot consolidated at suitable
temperatures to decompose the carbidiferous agent and result in the
formation of carbide and oxide particles, and to provide a powder compact
having a formable, substantially void-free mass. The compact is especially
suited for use in aerospace, automotive, electronic, wear resistance
critical components, and the like, which often encounter service
temperatures approaching 500.degree. C.
Inventors:
|
Das; Sontosh K. (Randolph, NJ);
Zedalis; Michael S. (Randolph, NJ);
Gilman; Paul S. (Suffern, NY)
|
Assignee:
|
Allied-Signal Inc. (Morris Township, Morris County, NJ)
|
Appl. No.:
|
433875 |
Filed:
|
November 9, 1989 |
Current U.S. Class: |
419/16; 75/233; 75/351; 148/437; 419/33; 419/45; 419/48; 419/60; 420/528 |
Intern'l Class: |
B22F 001/00; C22C 021/00; C22C 029/12 |
Field of Search: |
420/590,528,529,552
419/61,62,66-69
75/0.5 R,233,249
148/437-440,11.5 A,11.5 P
|
References Cited
U.S. Patent Documents
3591362 | Jul., 1971 | Benjamin | 75/0.
|
4594222 | Jun., 1986 | Heck et al. | 420/529.
|
4624705 | Nov., 1986 | Jatkar et al. | 420/528.
|
4627959 | Dec., 1986 | Gilman et al. | 419/61.
|
4722751 | Feb., 1988 | Akechi et al. | 75/232.
|
Primary Examiner: Dean; R.
Assistant Examiner: Koehler; Robert R.
Attorney, Agent or Firm: Buff; Ernest D., Fuchs; Gerhard H.
Claims
We claim:
1. A process for producing a composite having a metal matrix and a
reinforcing phase, comprising the steps of:
(a) forming a charge containing, as ingredients, a rapidly solidified
aluminum base alloy, a carbidiferous agent in an amount ranging from about
0.01 to 10 % by wt, and particles of a reinforcing material present in an
amount ranging from about 0.1 to 50 % by vol. of said charge;
(b) ball milling the charge energetically to mix the carbidiferous agent
within the aluminum matrix, and to enfold metal matrix material around
each of said particles while maintaining the charge in a pulverulent
state; and
(c) consolidating said charge to react the aluminum matrix with the
carbidiferous agent resulting in the formation of carbides and oxides, and
to provide a mechanically formable, substantially void-free mass.
2. A process as recited in claim 1, wherein said rapidly solidified
aluminum based alloy has a substantially uniform structure.
3. A process as recited in claim 2, wherein said rapidly solidified
aluminum based alloy is prepared by a process comprising the steps of
forming a melt of the aluminum based alloy and quenching the melt on a
moving chill surface at a rate of at least about 10.sup.5 .degree. C./sec.
4. A process as recited in claim 3, wherein said ball milling step is
continued until said carbidiferous agent and/or said particles are
enveloped in and bonded to said matrix material.
5. A process a recited in claim 4, wherein said consolidation step is
carried out at a temperature ranging from about 400.degree. C. to
600.degree. C., said temperature being below the solidus temperature of
said metal matrix.
6. A process as recited in claim 5, wherein said consolidation step
comprises vacuum hot pressing at a temperature ranging from about
450.degree. C. to 550.degree. C.
7. A process as recited in claim 3, wherein said rapidly solidified
aluminum based alloy has a composition consisting essentially of the
formula Al.sub.bal Fe.sub.a Si.sub.b X.sub.c wherein X is at least one
element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta,
"a" ranges from 2.0 to 7.5 at %, "b" ranges from 0.5 to 3.0 at %, "c"
ranges from 0.05 to 3.5 at % and the balance is aluminum plus incidental
impurities, with the proviso that the ratio [Fe+X]:Si ranges from about
2.0:1 to 5.0:1.
8. A process as recited in claim 7, wherein said rapidly solidified
aluminum based alloy is selected from the group consisting of the elements
Al-Fe-V-Si, wherein the iron ranges from about 2.0-7.5 at %, vanadium
ranges from about 0.05-3.5 at %, and silicon ranges from about 0.5-3.0 at
%.
9. A process as recited in claim 3, wherein said rapidly solidified
aluminum based alloy has a composition consisting essentially of the
formula Al.sub.bal Fe.sub.a Si.sub.b X.sub.c wherein X is at least one
element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta,
"a" ranges from 2.5 to 7.5 at %, "b" ranges from 0.75 to 9.0 at %, "c"
ranges from 0.25 to 4.5 at % and the balance is aluminum plus incidental
impurities, with the proviso that the ratio [Fe+X]:Si ranges from about
2.01:1 to 1.0:1.
10. A process as recited in claim 3, wherein said rapidly solidified
aluminum based alloy has a composition consisting essentially of about
2-15 at % from a group consisting of zirconium, hafnium, titanium,
vanadium, niobium, tantalum, erbium, about 0-5 at % calcium, about 0-5 at
% germanium, about 0-2 at % boron, the balance being aluminum plus
incidental impurities.
11. A process as recited in claim 4, wherein said carbidiferous agent is
selected from the group consisting of stearic acid, methanol, graphite,
and oxalic acid.
12. A process as recited in claim 4, wherein said particles are selected
from the group consisting of carbides, borides, nitrides, oxides and
intermetallic compounds.
13. A process as recited in claim 12, wherein said particles are selected
from the group consisting of silicon carbide and boron carbide particles.
14. A process as recited in claim 4, wherein said particles of reinforcing
material and said carbidiferous agent are substantially uniformly
distributed within said matrix material.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for improving the mechanical properties
of metals, and more particularly to a process for stabilizing an aluminum
composite having a rapidly solidified metal matrix and reinforcing phases
by incorporation of oxides and carbides through mechanical alloying.
2. Description of the Prior Art
An aluminum based composite is generally comprised of two components--an
aluminum alloy matrix and a hard reinforcing second phase. The composite
typically exhibits at least one characteristic reflective of each
component. For example, an aluminum based metal matrix composite should to
reflect the ductility and fracture toughness of the aluminum matrix and
the elastic modulus and thermal stability of the reinforcing phase.
Aluminum based metal matrix composites containing particulate
reinforcements are usually limited to ambient temperature applications
because of the large mismatch in higher temperature strength between the
aluminum matrix (low strength) and the particle reinforcement (high
strength). Another problem with aluminum based metal matrix composites is
that the dispersed strengthening phase is not stable at elevated
temperatures, and coarsens after excessive thermal exposure, which in turn
leads to a degradation of the materials' mechanical properties. Another
problem with aluminum based metal matrix composites is the difficulty of
producing a bond between the matrix and the reinforcing phase. To produce
such a bond, it is often times necessary to vacuum hot press the material
at temperatures higher than the incipient melting temperature of the
matrix. It has been proposed that this technique be avoided by
mechanically alloying the matrix with the addition of particulate
reinforcements. This procedure, referred to as solid state bonding,
permits the reinforcing phase to be bonded to the matrix without heating
the material to a temperature above the solidus of the matrix. Moreover,
it has been further proposed that mechanical alloying be performed with
the addition of a carbidiferous agent, e.g., stearic acid, which will
become uniformly dispersed within the aluminum base matrix powder during
processing, and subsequently will decompose during vacuum hot degassing
and/or hot consolidation, e.g., extrusion, forging, rolling, and form
carbides and oxide particles dispersed within the matrix.
Although carbidiferous agents, said to be necessary for the mechanical
alloying of aluminum base alloys, can become constituents in the final
product (see, for example U.S. Pat. No. 4,627,959), prior art teachings
suggest that the resulting Al.sub.4 C.sub.3 particles are not suitable for
use at temperatures greater than 100.degree. C. Specifically, it has been
taught that upon exposure to temperatures above 100.degree. C., age
hardened structures and/or work hardened structures tend to soften. At
higher temperatures the dispersion of Al.sub.4 C.sub.3 in the alloy is
said to coarsen, thus lessening the contribution of carbide to the
strength of the alloy. In consequence, aluminum base alloys of the prior
art as produced by mechanical alloying are said to be generally unsuitable
for use in the temperature range of 100.degree. C. to 500.degree. C. These
aluminum carbides and oxides will provide further reinforcements in
mechanical and physical properties at ambient and elevated temperatures.
Prior processes in which aluminum based alloys and/or metal matrix
composites are mechanically alloyed by means of solid state bonding are
disclosed in U.S. Pat. Nos. 4,722,751, 4,594,222 and 3,591,362.
For the above reasons, in use of a carbidiferous processing aid, it has
been proposed (see U.S. Pat. No. 4,624,705) that strong carbide formers
such as titanium be added to produce in the final alloy carbides more
thermally stable than Al.sub.4 C.sub.3 at temperatures in excess of
100.degree. C.
SUMMARY OF THE INVENTION
The present invention provides a process for producing a stabilized
aluminum composite suitable for use at temperatures approaching
500.degree. C. wherein a strong carbide former is not needed. The
composite produced by the process has a rapidly solidified metal matrix
and reinforcing phases. Oxides and carbides are incorporated within the
metal matrix by mechanical alloying to improve thermal stability and
increase elevated temperature strength and creep resistance of the
composite. The ability to mechanically alloy the rapidly solidified
material is not dependent on the presence of a carbidiferous agent.
Advantageously, the desired volume friction of resulting carbides and
oxides can be engineered into the material without the restrictions
heretofore required to control the mechanical alloying process.
More specifically, the invention provides a process for producing a
composite material comprising the steps of forming a charge containing, as
ingredients, a rapidly solidified aluminum alloy, a carbidiferous agent in
an amount ranging from about 0.01 to 10 wt. percent and particles of a
reinforcing material such as a hard carbide, oxide, boride, carbo-boride,
nitride or a hard intermetallic compound, the reinforcing material being
present in an amount ranging from about 0.1 to 50 % by volume of the
charge, and ball milling the charge energetically to mix the carbidiferous
agent within the aluminum matrix, and to enfold metal matrix material
around each of the reinforcing particles while maintaining the charge in a
pulverulent state. In this manner there is provided a strong bond between
the matrix material and the surface of the reinforcing particle. Upon
completion of the ball milling step, the resultant powder is hot pressed
or sintered using conventional powder metallurgical techniques, to react
the aluminum matrix with the carbidiferous agent resulting in the
formation of carbides and oxides, and to form a powder compact having a
mechanically formable, substantially void-free mass. The compressed and
treated powder compact is then mechanically worked to further react the
carbidiferous agent and the aluminum matrix, and to increase its density
and provide engineering shapes suitable for use in aerospace components
such as stators, wing skins, missile fins, actuator casings, electronic
housings and other wear resistance critical parts, automotive components
such as piston heads, piston liners, valve seats and stems, connecting
rods, cam shafts, brake shoes and liners, tank tracks, torpedo housings,
radar antennae, radar dishes, space structures, sabot casings, tennis
racquets, golf club shafts and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood and further advantages will
become apparent when reference is made to the following detailed
description of the preferred embodiment of the invention and the
accompanying drawings in which:
FIGS. 1A and 1B are transmission electron micrographs of a rapidly
solidified aluminum based iron, vanadium and silicon containing alloy
ribbon and a rapidly solidified aluminum based titanium containing alloy
ribbon produced by melt spinning;
FIGS. 2A and 2B are photomicrographs of an aluminum based iron, vanadium
and silicon containing alloy and an aluminum based titanium containing
alloys fabricated by conventional ingot casting; and
FIG. 3 is a photomicrograph of a rapidly solidified aluminum based titanium
based containing alloy powder having about 8 % by volume aluminum carbide
particles substantially uniformly distributed therein in accordance with
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The aluminum base, rapidly solidified alloy appointed for use in the
process of the present invention has a composition consisting essentially
of the formula Al.sub.bal Fe.sub.a Si.sub.b X.sub.c wherein X is at least
one element selected from the group consisting of Mn, V, Cr, Mo, W, Nb,
Ta, "a" ranges from 2.0 to 7.5 at %, "b" ranges from 0.5 to 3.0 at %, "c"
ranges from 0.05 to 3.5 at % and the balance is aluminum plus incidental
impurities, with the proviso that the ratio [Fe+X]:Si ranges from about
2.0:1 to 5.0:1. Examples of the alloy include
aluminum-iron-vanadium-silicon compositions wherein the iron ranges from
about 2.0-7.5 at %, vanadium ranges from about 0.05-3.5 at %, and silicon
ranges from about 0.5-3.0 at %.
Another aluminum base, rapidly solidified alloy suitable for use in the
process of the invention has a composition consisting essentially of the
formula Al.sub.bal Fe.sub.a Si.sub.b X.sub.c wherein X is at least one
element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta,
"a" ranges from 1.5 to 7.5 at %, "b" ranges from 0.75 to 9.0 at %, "c"
ranges from 0.25 to 4.5 at % and the balance is aluminum plus incidental
impurities, with the proviso that the ratio [Fe+X]:Si ranges from about
2.01:1 to 1.0:1.
Still another aluminum base, rapidly solidified alloy that is suitable for
use in the process of the invention has a composition range consisting
essentially of about 2-15 at % from a group consisting of zirconium,
hafnium, titanium, vanadium, niobium, tantalum, erbium, about 0-5 at %
calcium, about 0-5 at % germanium, about 0-2 at % boron, the balance being
aluminum plus incidental impurities.
Rapid solidification of those alloys is accomplished in numerous ways,
including planar flow or jet casting methods, melt extraction, splat
quenching, atomization techniques and plasma spray methods. These metal
alloy quenching techniques generally comprise the step of cooling a melt
of the desired composition at a rate of at least about 10.sup.5 .degree.
C./sec. Generally, a particular composition is selected, powders or
granules of the requisite elements in the desired portions are melted and
homogenized, and the molten alloy is rapidly quenched on a chill surface,
such as a rapidly moving metal substrate, an impinging gas or liquid.
When processed by these rapid solidification methods the aluminum alloy is
manifest as a ribbon, powder or splat of substantially uniform structure.
This substantially uniformly structured ribbon, powder or splat may then
be pulverized to a particulate for further processing. By following this
processing route to manufacture the aluminum matrix, the resultant
microstructure is significantly refined and homogeneous. Such
microstructural improvements typically result in improved ambient and
elevated temperature strength, fracture toughness and ductility when
compared to alloys of similar composition fabricated by conventional ingot
casting or other techniques wherein the molten metal cools at relatively
slow rates. The aluminum matrix material must be provided as a particulate
that can range in size from 0.64 cm in diameter down to less than 0.0025
cm in diameter. For the purposes of this specification and claims the term
"hard", as applied to particles which may form the reinforcing phase of
the resultant composite shall generally imply (1) a scratch hardness in
excess of 8 on the Ridgway's Extension of the MOHS' Scale of Hardness, and
(2) an essentially nonmalleable character. However, for the aluminum
matrices of this invention somewhat softer reinforcing particles such as
graphite particles may be useful. Hard particles useful in the process of
this invention include filamentary or non-filamentary particles of silicon
carbide, aluminum oxide and/or aluminum hydroxide (including additions
thereof due to its formation on the surface of the aluminum matrix
material), zirconia, garnet, cerium oxide, yttria, aluminum silicate,
including those silicates modified with fluoride and hydroxide ions,
silicon nitride, boron nitride, boron carbide, simple or mixed carbides,
borides, carbo-borides and carbonitrides of tantalum, tungsten, zirconium,
hafnium and titanium, and intermetallics such as Al.sub.3 Ti, AlTi,
Al.sub.3 (V, Zr, Nb, Hf and Ta), Al.sub.7 V, Al.sub.10 V, Al.sub.3 Fe,
Al.sub.6 Fe, Al.sub.10 Fe.sub.2 Ce, and Al.sub.12 (Fe, Mo, V, Cr,
Mn).sub.3 Si. Such particles of reinforcing material may be present in an
amount ranging from about 3 to 25% by volume, and preferably 5 to 15% by
volume. In particular, because the present invention is concerned with
aluminum based composites that possess a relatively low density and high
modulus, silicon carbide and boron carbide are desirable as the
reinforcing phase. However, other particulate reinforcements may prove to
form superior matrix/reinforcement bonds. Also, the present specification
is not limited to single types of reinforcement or single phase matrix
alloys.
As used herein, the term "carbidiferous agent" means carbon based material
including compounds and mixtures such as stearic acid, methanol, oxalic
acid, etc. as well as carbonitrides and carbides containing free carbon.
The term "energetic ball milling" in the context of the present
specification and claims means milling at prescribed conditions where the
energy intensity level is such that the hard reinforcing phase and/or the
carbidiferous agent is optimately kneaded into the aluminum matrix. As
used herein, the phrase "prescribed conditions" means conditions such that
the ball mill is operated to physically deform, fracture, cold weld and
re-fracture the matrix metal alloy powder so as to distribute the
reinforcing phase and/or carbidiferous agent therewithin. The phrase
"optimately kneaded", as used herein, means that the reinforcing phase
and/or carbidiferous agent is distributed more uniformly than the
distribution produced by simple mixing or blending, and approaches a
substantially homogeneous distribution of reinforcing material and/or
processing control agent within the matrix. Energetic ball mills include
vibratory mills, rotary ball mills and stirred attritor mills.
After the ball milling step is completed, the resultant powder is compacted
alone or mixed with additional matrix material, under conditions to
promote the decomposition of the carbidiferous agent, and formation of
carbides and oxides. Consequently, the resultant composite compact is
vacuum hot pressed or otherwise treated under conditions such that the
carbidiferous agent decomposes and reacts with the aluminum matrix, and
that no significant melting of the matrix occurs. Generally, the
consolidation step is carried out at a temperature ranging from about
400.degree. C. to 600.degree. C., and preferably from about 450.degree. C.
to 550.degree. C., the temperature being below the solidus temperature of
the metal matrix. The Al-Fe-V-Si alloy composite containing a
carbidiferous agent and silicon carbide reinforcements may be canless
vacuum hot pressed at a temperature ranging from 435.degree. C. to
500.degree. C. and more preferably from 450.degree. C. to 475.degree. C.,
followed by forging or extrusion.
Those skilled in the art will appreciate that other time/temperature
combinations can be used and that other variations in pressing and
sintering can be employed. For example, instead of canless vacuum hot
pressing the powder can be placed in metal cans, such as aluminum cans
having a diameter as large as 30 cm or more, hot degassed in the can,
sealed therein under vacuum, and thereafter reheated within the can and
compacted to full density, the compacting step being conducted, for
example, in a blind died extrusion press. In general, any technique
applicable to the art of powder metallurgy which does not involve
liquefying (melting) or partially liquefying the matrix metal can be used.
Representative of such techniques are explosive compaction, cold isostatic
pressing, hot isostatic pressing and direct powder extrusion. The
resultant billet can then be worked into structural shapes by forging,
rolling, extrusion, drawing and similar metal working operations.
EXAMPLE I
Ten kilogram batches of aluminum alloys of the compositions
aluminum=balance, 4.06 at % iron, 0.70 at % vanadium, 1.51 at % silicon
(hereinafter designated Alloy A), and aluminum-balance, 4.7 at % titanium
(hereinafter designated Alloy B) were produced by planar flow casting.
Transmission electron photo-micrographs of the rapidly solidified ribbon
are shown in FIGS. 1A and 1B, respectively. The
aluminum--iron--vanadium--silicon base alloy microstructure is composed of
a microcellular network of aluminum intermetallic compound particles,
Al.sub.13 (Fe, V).sub.3 Si, uniformly distributed in the aluminum solid
solution network. The aluminum--titanium base alloy microstructure is
composed of titanium-rich cell boundaries, within which is a uniform
distribution of fine aluminum intermetallic compound particles, Al.sub.3
Ti.
For comparison, light photomicrographs of these two alloys made by
conventional ingot casting are shown in FIGS. 2A and 2B respectively. The
dispersed phases present in these alloys are observed to be much coarser
and less uniformly distributed than the dispersed phases formed in planar
flow cast alloys.
EXAMPLE II
A five gram sample of -40 mesh (U.S. standard sieve) powder of Alloy A was
added to 0.10 grams of Nopcowax.RTM., i.e., stearic acid. The sample was
processed by pouring the powders into a Spex Industries hardened steel
vial (Model #8001) containing 31 grinding balls. Each of the balls had a
diameter of about 0.365 cm and was composed of Alloy SAE 52100 steel. The
filled vials were then sealed and placed into a Spex Industries 8000 mixer
mill. The powder batch containing about 8 vol. % Al.sub.4 C.sub.3
particles was then processed for 240 min. The processing procedure
described above provides a composite aluminum base alloy with silicon
carbide particulate in the form of powder particle that exhibit a
substantially uniform dispersion of the carbidiferous agent and the
reinforcement, and strong aluminum metal to aluminum carbide bonding. A
photomicrograph of said composite powder particles containing 8 vol. %
Al.sub.4 C.sub.3 particulate that have been processed for 240 min. is
shown in FIG. 3.
Having thus described the invention in rather full detail, it will be
appreciated that such detail need not be strictly adhered to but that
various changes and modifications may suggest themselves to one skilled in
the art, all falling within the scope of the invention as defined by the
subjoined claims.
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